Set Size Of Object To Perfectly Align 64x64 Image
If it's sufficient to align the UV islands to the pixel grid of the image, then you can snap selected vertices to the pixel grid with Shift+S > Snap Selected to Pixels.

Additionally, there's the option to snap the vertices with every transformation by activating UVs > Snap to Pixels in the menu of the UV/Image Edtior.



If the proportions within your texture are important, you should make sure to align and scale all islands so that they approximately lie on the grid before you snap them to it. Otherwise your islands will be distorted: